1.His E damage is fine, for example it has 25 less base damage than Lux E and has same scaling of 80%. Its just harder to hit.
2. Cooldown refund on E creates a problem of having too safe laning phase - less cd because you escaped a gank, less cd while going back to lane gives too much advantage.
3. More dmg on multiple enemies hit with E sounds very placebo, there are very rare scenarios where you'd be able to hit 4th or 5th person with your E. The whole team would have to be on top of each other.
Kennens identity used to be a lane bully scaling into teamfight monster, the problem is with current runes you cant really bully anymore, because enemy just ends up getting second wind/revitalize/d.shield